Add end-to-end streaming pipeline integration test

[demoray]

Adds a unit test in src/upload/stream.rs that drives the full streaming pipeline end-to-end: Snapshot::create_to_writer → BlockBlobStream → in-memory FakeStager.

Existing tests cover BlockBlobStream against FakeStager directly and Snapshot in isolation, but nothing wires the three together. This test fills that gap.

Coverage:

• Both Format::Lime and Format::AvmlCompressed.
• Committed block order: exactly one commit_block_list call, IDs sorted ascending.
• Zero-only ranges are elided (no header, no staged block).
• Trailing partial block is staged on shutdown.
• Reconstructed committed bytes round-trip through Image::convert_block and reproduce the original payload.

Lives as a unit test (rather than under tests/) to reuse the crate-private BlockBlobStream::with_stager and FakeStager without expanding the public API.